The present invention relates to a method for diffusing an impurity into semiconductor wafers by introduction of a gas containing the impurity from one end of a horizontal diffusion furnace.
A horizontal diffusion furnace used for diffusion of an impurity into semiconductor wafers (hereinafter merely referred to as "wafers") generally has heating means separated into three heaters. That is, the heater means has a front heater provided at the opening side of a core pipe through which the wafers are introduced, a center heater provided at the center portion of the core pipe on which the wafers are mounted, and a rear heater provided at the side through which an impurity gas is introduced. The heaters are arranged so that the respective temperatures thereof can be controlled separately. The temperature of a center heater region is set to a range of, for example, .+-.1.0.degree. C. with respect to the set temperature thereof, by adjusting the respective values of power for the front and rear heaters.
After a plurality of wafers mounted on a quartz boat has been introduced into the center heater region in the horizontal diffusion furnace which has been set to a uniform temperature as described above, an impurity gas such as phosphorus oxychloride (POCl.sub.3), boron (BBr.sub.3) or the like is introduced from the closing side (rear heater side) of the core pipe to carry out diffusion of the impurity into the semiconductor wafers.
However, the aforementioned conventional impurity diffusing method has the following problem.
That is, an impurity introduced from the rear side of the horizontal diffusion furnace is deposited on the wafers successively in order of starting from the rear-side wafers, so that the concentration of the impurity gas decreases as the impurity gas goes toward the front side. As is obvious from FIG. 5, the quantity of impurity diffused into wafers decreases as the impurity concentration of the introduced gas decreases. From this reason, the conventional impurity diffusing method has a problem in that the quantity of impurity diffused into wafers decreases in the order of arrangement of the wafers from the rear side to the front side, and that this results in scattering in quantity of impurity among the wafers.
As a measure to overcome the aforementioned problem, it may be considered to reduce the number of wafers introduced into the horizontal diffusion furnace to thereby suppress the variations in concentration of the impurity gas. However, there arises another problem in that treatment efficiency in the diffusing step is lowered.